This invention relates to heat exchangers for harsh environments and in particular to modular heat exchange tubes and tube sheets of a novel and advantageous composition and construction.
High temperature industrial furnaces often operate very inefficiently due to exhaust gas thermal losses. In the aluminum industry, for example, stack losses for some furnaces are estimated to be as high as seventy-five percent. Major waste heat losses are associated with exhaust gases of aluminum remelt furnaces and, as well, in glass melting and steel soaking and reheat operations. The exhaust gases in such applications have temperatures in the range of 1500.degree. to 3000.degree. F. Moreover, the high-temperature gases contain surface-fouling particulates and highly corrosive contaminants which may readily degrade exhaust flue construction materials. The particulates tend to clog and foul the surfaces of heat exchangers inserted in the flue for heat recovery purposes.
Thus, efficient recovery of waste heat from high-temperature (greater than 1500.degree. F.) industrial, dirty waste gas streams of glass melting furnaces, aluminum remelt furnaces and steel soaking and reheat furnaces represent a high-priority objective.
In the past, metallic exchangers have been used to recover waste heat. Metals cannot, however, usually retain required strengths at material temperatures that exceed 1500.degree. F. Although the addition of exotic alloying agents may extend the retention of strength to about 1900.degree. F. in the environment of a relatively clean flue gas stream, hot corrosive contaminants will reduce the durability of such metals. Thus, conventional metallic heat exchangers cannot survive for extended periods in these high-temperature, highly corrosive environments unless a significant dilution of the gas stream to a lower flue gas temperature is effected which, in any event, reduces the efficiency of the heat recovery process.
Ceramic materials offer the potential of allowing significantly higher material temperatures while resisting attack by many of the corrosive and surface-fouling contaminants that are present in industrial waste gas streams. Previous attempts to recover waste heat with ceramic heat exchangers have met with limited success. The major problems have been leakage through joints, such as tube-to-tube sheet joints, and thermally-induced cracks. In addition, many of the ceramic-based heat exchanger designs have narrow flow passages which are prone to clogging in fouling environments resulting in high pressure drops, especially on the hot flue gas side. Other difficulties encountered in making ceramic heat exchangers include problems in making complex shapes, including thin-walled sections while simultaneously achieving high resistance to corrosion and tube cracking caused by thermal shock.
Accordingly, a need has been recognized for advanced heat recovery heat exchangers having durable construction materials that are compatible with the waste stream constituents, in temperature ranges of 1500.degree. F. to 3000.degree. F., that resist fouling and are easily cleaned, and which provide commercially viable heat exchange rates and effectiveness while being physically sized to allow retrofitting in existing facilities.